Microelectronic devices have become ubiquitous in our society and touch all aspects of our lives. They are the driving force for many technologies. Within the past two decades, the manufacture of microelectronic devices has burgeoned into a multibillion dollar business. This business is specifically concerned with the various types of microelectronic devices available, referred to as chips. Developments in past years have increased the performance and design of these devices from initial introductory models which contained only a few thousand transistors per chip to current models which contain millions of transistors per chip in approximately the same area. The increased performance requirements and the higher density of transistors on these chips have caused greater demand for improvements on current manufacturing methods.
Early methods for preparation of devices consisted of use of photolithographic techniques wherein a photoresist was placed or coated on a substrate, usually silicon, imaged with light and developed into desired patterns with suitable developers. The resulting image was then used as a selective mask in subsequent operations such as ion implanting, patterning of the underlying substrate, metal plating or various other required steps for the production of microelectronic devices. These earlier photoresist applications presented a number of problems including poor photoresist coating of the substrate wafer, lifting-off of photoresist patterns from devices, and subsequent pattern loss due to portions of photoresist being carried off by developer when the developer undercut the resist. Undercutting is a deleterious process wherein an aqueous or organic developer migrates along the surface of a polar substrate and causes a photoresist to lose its adhesive ability.
Hexamethyldisilazane (HMDS) was found to be a significant solution to many of these problems. As a pretreatment to a photoresist application, HMDS when applied to substrates was found to promote better photoresist coating on the substrates, reduce undercutting and prevent photoresist film lift-off during development. The noticeable improvement in coating properties is the result of the reaction of HMDS with water molecules, hydrogen bonded to the substrate surface, and the subsequent reaction of HMDS with hydroxide and oxide groups present on the surface. The HMDS reaction with the water molecules produces ammonia and trimethylsilanol. The subsequent reaction with the hydroxyl and oxide groups produces a trimethylsiloxy substituted surface. Incorporating trimethylsilyl groups on the surface reduces surface energy and provides a monomolecular organic coating which is compatible with organic photoresists. The hydrophobic trimethylsilyl groups repel polar groups such as water and aqueous developers, preventing undercutting at the substrate-photoresist interface. HMDS was rapidly employed by all device manufacturers due to its delivery of an increase in yield.
Further improvements to the early priming techniques, which comprised applying liquid HMDS or HMDS diluted in various solvents, developed in subsequent years through the application of HMDS in a vapor form. The methods comprised placing the substrate in an oven at a reduced pressure and treating the substrate with HMDS vapor. These methods gave more consistent coverage and were more efficient regarding reaction time and required materials. Today, vapor priming of wafers is a method of choice in the manufacture of these high density devices.
Vapor priming, while superior to liquid priming, requires more time for the actual priming operation. Recent methods of vapor priming include utilizing state-of-the-art, in-line track priming in which a substrate wafer is placed on a track and transported to an area where heat and vacuum are applied. A device covers the wafer, and a vacuum is applied as heat is supplied to the substrate. HMDS vapor is introduced into the area surrounding the wafer when the proper vacuum is achieved. The vacuum is broken and the wafer is transported to the next operation. A successful vapor priming step enables acceptable photoresist films, or other organic-based films, to be applied in subsequent steps. An acceptable photoresist film is a continuous, uniform film that does not exhibit pin-holes, edge pullback, beading, lifting and/or significant undercutting during development.
A measurement of the degree of surface silylation achieved during vapor priming is effectively quantified by the surface contact angle as measured by a goniometer. Higher degrees of silylation cause a reduced surface energy resulting in higher contact angles for a water bead placed upon a surface. A contact angle in the range of from about 65.degree. to 75.degree. is suitable for good coating characteristics in most applications. For some substrates, however, it is desirable to reach a higher degree of silylation for good coating characteristics as indicated by contact angles of up to 80.degree.. The contact angle measurements are significant in that they provide proof of the level of silylation that is obtained with the various techniques described. The higher the contact angle, the more silylation has taken place. Therefore, a high contact angle indicates a greater number of trimethylsilyl groups bonded to the substrate surface.
Typical in-line vapor track priming of a wafer using HMDS may require in excess of two minutes in order to obtain sufficient surface silylation and to allow for successful photoresist coating. The subsequent process in the track, photoresist coating, typically requires about one minute or less. Therefore, a significant portion of the time required to produce the finished device is consumed in the vapor priming operation. The priming step may limit otherwise faster operations. A high density device may use up to 20 to 25 masking steps or levels, each of which requires prior photoresist application for the HMDS priming. Attempts to decrease this priming time by increasing substrate temperatures, increasing the amount of HMDS vapor, and changing pressure have been ineffectual in reducing the amount of time for priming.
Other silylation agents are known which could impart the trimethylsilyl moiety to the substrate surface. Notable among them is trimethylsilyldiethylamine (TMSDEA). This compound is known to rapidly and completely silylate substrate surfaces at low temperatures. However, the material is so reactive that control of the extent of surface silylation is not easily achieved with over-silylation a frequent occurrence. The over-silylation of the surface promotes the wetting of a photoresist and a blistering or "blowoff" of the photoresist during imaging. The latter is caused by a high degree of silylation which acts as a release agent with nitrogen gas evolved during exposure, lifting off sections or pieces of the photoresist film.